Bypass adjustment screws are typically used in conjunction with a bypass passage or valve that may be included in a variety of process systems such as, for example, volume boosters. A volume booster is often used to increase the stroking speed of control valves or other devices. The volume booster may be activated based on different factors such as the size of a pressure input from a positioner, attaining a requisite pressure differential across the volume booster, and/or a position of a bypass adjustment screw relative to the bypass passage. Activating the volume booster causes the volume booster to deliver a fluid flow to an actuator to assist in changing the position of a valve or other devices coupled to the actuator.
A bypass adjustment screw of a volume booster can be adjusted to restrict a bypass passage which, in turn, changes the magnitude of a pressure input that activates the volume booster. For example, if the bypass adjustment screw substantially blocks or restricts the bypass passage, a relatively small pressure input will activate the volume booster because the requisite pressure differential is attained across the volume booster signal diaphragms at a relatively low input signal. Alternatively, if the bypass adjustment screw is adjusted so that the bypass passage is substantially unblocked or unrestricted (e.g., open), then a relatively large pressure input will be needed to activate the volume booster because the requisite pressure differential is attained across the volume booster at a relatively higher input signal.
Known bypass adjustment screws have a conical head that can be adjustably moved toward or away from a seat of the bypass passage. In particular, as the bypass adjustment screw is adjusted (e.g., turned), the conical head moves toward or away from the seat and, thus, changes the fluid flow rate and/or the amount of fluid that is allowed to flow through the bypass passage without activating the volume booster. The opening between the conical head and the seat typically provides a quick-opening flow characteristic. Such a quick-opening flow characteristic provides a maximum change in flow rate at low travel (i.e., a small adjustment of the bypass adjustment screw) and, thus, known bypass adjustment screws have a tendency to overshoot and/or undershoot the desired flow rate (e.g., provide too little or too much fluid flow through the bypass passage). As a result, the adjustment time required to calibrate the bypass adjustment screw may be increased significantly.